1. Field of the Invention
The present invention relates to metal composites for fuel cells, which have a core cladded with a corrosion resistant metal and have through-holes formed therethrough, and fuel cell bipolar plates fabricated from such a metal composite. The present invention also relates to fabrication methods for the metal composites.
2. Description of Related Art
Conventionally, metal composites formed by laminating and bonding a plurality of metals are used in various fields. In particular, materials having a Ti (titanium) surface layer are being developed for use in fuel cell bipolar plates exposed to harsh corrosive environments. Examples of prior patent documents which disclose a fuel cell bipolar plate fabricated from a material having a Ti surface layer includes, e.g., JP-A-2006-210320 and JP-A-2005-158441.
The Ti is a typical corrosion resistant metal. However, corrosion resistant metals are very expensive and therefore cannot be used freely for bipolar plates of consumer fuel cells. To address this problem, the present inventors are developing bipolar plates fabricated from a metal composite made of an inexpensive core material cladded with a corrosion resistant metal on both surfaces (e.g., JP-A-2006-210320).
Generally, in fuel cells, the fuel gas and oxidant gas (e.g., air) used for generating electricity are humidified (e.g., concentrated methanol is humidified (diluted) with water generated at the air electrode) in order to improve the power generation characteristics. These gases are supplied to each MEA (Membrane Electrode Assembly) via through-holes formed in the bipolar plates. In other words, each bipolar plate is required to be provided with through-holes, which are part of the fuel and oxidant gas conduits.
A problem with conventional metal composites for fuel cells is that the hole wall of the core layer region of the through-hole is exposed to the ambient environment, and therefore dissolution (corrosion) of the core material can occur due to dew condensation or a pH change caused by impurity ions contained in the oxidant gas. Furthermore, when an Al (aluminum) is employed as the core metal in order to reduce the weight of a bipolar plate, more reliable anti-corrosion protection is needed because the Al is far less resistant to such core dissolution than a stainless steel.
A method for covering exposed core surfaces is disclosed, for example, in the above-mentioned JP-A-2005-158441 in which, in order to suppress corrosion of the core metal of the bipolar plate, the through-hole wall and plate surfaces surrounding the hole are adhesively covered by a film coating. However, the method according to the above JP-A-2005-158441 covers each through-hole individually, and therefore can cause an increase in the number of components and the manufacturing cost. Further, in this method, the resin film can block the through-hole, or can thicken the portion of the bipolar plate surrounding each through-hole, thus possibly incurring assembly difficulty.